Hoisting at night for that?
They might not have known her injuries were not life threatening when they elected to winch - the decision might have been made purely because her location/situation had the potential to be life-threatening if she had been left there.
However this is either an equipment failure or a serious procedural error for the rescuer to have fallen off the winch whilst the casualty remains attached - wonder if they had a pin in the winch hook?
It would be normal for the winchman(woman) to be last on the hook and, if the keep/gate on the hook opened during the cabin entry, it is possible for the karibiner (or whatever attachment method they use) to slip out.